Duplex radio system



Fgb. s; 1927, 1,616,892

| ESPENSCHIED ET m. v

I DUPLEX RADIO SYSTEM I Filed Aug. 18. 1922 Mb ATTORNEY Patented Feb. 8,1927.

m starts rare-sea rarest caries...

LLOYD ESPENSOI-IIED, OF HOLLIS, NEXV YORK, AND DE LOSS K. ."Mi'zrRCCIh'l', OF ORANGE,

NEW JERSEY, ASSIGNOR-S TO AMERICAN TELEPHONE AND A CORPORATION OF NYORK.

TELEGRAPH COMPANY,

DUPLEX RADIO SYSTEM.

This invention relates to an improvement in duplex radio signaling, andin particular has for its purpose that of enabling a better and morecomplete separation of the transmitted and the received waves.

In duplex radio signaling, especially in those cases where oneantenna isused for the transmitted and received signals, the enormous value of theratio of the transmitted power to the received power makes the problemof separation an extremely difficult one. In this invention we proposeto use one frequency for transmitting and a slightly different frequencyfor receiving. Heretofore, i such cases, reliance has been placed forseparation mainly on the discrimination which is afforded by tunedcircuits. However, if the tuning of such circuits is made too sharp andselective, the range of frequencies transmitted by the respectivecircuits is too narrow for practical signaling purposes. This isespecially true in the case of radio telephony, for in that case theband of frequencies transmitted or received must be at least as wide asthe speech range of frequencies which is to be transmitted.

Normally this speech band is about 2,500.

Wide, and if the carrier frequency is, for example, 100,000, then thetransmitted or received oscillations will vary from about 97,500 toabout 102,500. This means that the antenna and associated circuitsshould not be tuned too sharply; otherwise import tant parts of thesignal will be substantially eliminated. The usual .method heretoforefor obtaining suflicient width for these circuits is to refrain from toosharp tuning, either of the antenna or its associated circuits. But inflattening the tuning of the circuits, discrimination is lost so far asfrequencies outside of the desired range is concerned. As a result, itis extremely difficult to obtain satisfactory separation of thetransmitted and the received oscillations. Conditions, so far as thisdiscrimination or separation is concerned, may, of course, be improvedby separating the frequencies of transmission and reception, but suchseparation means that a larger portion of available frequencies isrequired for one signaling channel. One object of this invention is tobring the transmitted and the received frequencies closer together andstill satisfactory separation.

In this invention we propose to makeuse of the peculiar characteristicsof wave filters which have the property of transmitting or suppressingcertain bands of" frequencies, and which at the same time enable 'asharp cut-off at any desired point. Such wave filters have been usedheretofore in radio and other circuits in various places, but in thisinvention we propose to use them in combinations to bring out theirinherent selective properties to the best advantage.

The invention will be better understood by reference to the accompanyingdrawings, in which Figure 1 shows one form of circuit for duplexsignalingyFig. 2 shows a modificationthereof; andFigsB to 3 show variousforms of-Wave filters with their characteristics which may be used inone place or another as disclosed hereinafter.

Referring more particularly to Fig. 1, there is shown an antenna 5 whichis used both for transmission and reception. The first of these may, forexample, take .place on a'frequency of 525,000 and the second'on afrequency of, say, 500,000. This antenna may be tuned with sufficientflatness to be sensitive to both of these frequencies with theirassociated side bands, or it may be given two degrees of freedom, one ofthese corpermit of I responding to the transmission frequency f and theother to the reception frequency, for both of these frequencies thetuning being sufiici-ently broad for the passage of the necessary bandof frequencies required for signaling purposes; Associated with thisantenna is a circuit adapted to take 0& the received oscillations orimpress the transmitted oscillations, this circuitfin turn beingassociated with some remote subscribers station,the message coming inand going out this filter the circuit leads to an amplifier I 10 whichmay be of any suitable form but is preferably of the thermionic vacuumtube type. A radiofrequency oscillation generator 12'is arranged toimpress oscillations on a modulating device 14, which latter ispreferably of the thermionic vacuum tube type. The amplified speechsignals are also, by means of the transformer 15, impressed on the inputcircuit of the device 1a and in series with the oscillations from thegenera tor 12. The device 141- is a modulator by virtue of its distortedor'curved characteristic and will contain in its output circuit, amongother things, a modulated wave on a carrier of 525,000 cycles. If s, betaken to represent the variable frequency of the speech message to betransmitted, the modulated' wave will consist of three components Whosevalues will be h, f -l-s' and f,s where f, represents the carrierfrequency which is here taken as 525,000. These modulated waves will nowbe impressed upon and radiated" from the antenna 5 by virtue of atransformer coupling including the coils 17 and 18: Thecondenser 19 maybe used in I addition to the coil 17 for the purpose of tuningjthiscircuit to any desired frequency. Received signals consisting of somecarrier frequency 72-, with the corresponding modulated side band-s f +sand f s will be received by the antenna and impressed on the outputcircuit of the modulating device 1 1. By virtue of its curvedcharacteristic, this device will also act as a demodulator to givecertain" frequencies including, among others, sum and-differencefrequencies of the received signals and the signals to be transmitted.If, forexample, the person at the remote'end' 0f the line L is silent,there will be present in the output of the device 1 L oscillations offrequency f,. These, combining with the incoming signals, will give thedifference frequencies f f and 7" f is,. In the case assumed, f,;, hasthe value of 25,000 but this may be made an y value de sired, althoughwe prefer to have it of fairly low ultra-audio frequency, for reasonswhich will appear later. For convenience, this intermediate frequencyf,,f will be designated by f,. Associated with the circuit of the device1 L by means of a transformer 20 is a circuit which leads to a detectingdevice. This circuit comprises first a band exclusion'filter 22 whichhas the property of excluding anv band of frequenciesfor which it may beadjusted, and: in this case it is adjusted to. exclude frequen cies inthe neighborhood of f and 7", but in such amanner as to permit freelythe passage-of the intermediate frequency f and its associated sidebands. Following this, there is inserted a band filter 23 which passesthe intermediate frequency and its side bands but is adjusted to cut offas sharply as possible rr-bo h sid s; The: cui ll W le d to an amplifier24, then to a second band filter 25 of the same properties as the filterThe output of the filter 25 is impressed on a detecting device 26 whichmay be of any desired form, although again we prefer the thermionicvacuum tube type. The output of this device 26 will contain a pluralityof different oscillations among which will be the message 8 and topermit the ready passage of this message but the suppression of allother frequencies, there is now inserted in the line a filter27,preferably of the lowpass type, allowing all frequencies below, say,2500 to pass. From here the circuit goes to the conjugate terminals ofthe coil 7, thus permittinga message to be transmitted to the remotespeaker.

In the description thus far we have spoken of the use of different typesof filters. Such filters are described fully in the patents to Campbell,No. 1,227,113, May 22,1917, and No. 1,493,600, dated May 13, 1924:.iVithout desiring to go into a detail-ed explanation of these, referencemay be made to Figs. 3 to 3, which willillustrate the characteristics ofthese filters. If the filters for example, be constructed of a series ofelements consisting of an inductance in series and a capacity C inshunt, the circuit will'have the properties of a low pass filter, thatis, it will permit all frequencies below a certain value to pass butwill suppress all frequencies above that value as indicated in thecharacteristic curve of Fig. 3 The sharpness of the cut-off will beincreased by using inductances and capacities of very low resist ancevalues, andit is the intention in this in vention that filters withsharp cut-off characteristics shall be used. By increasing thecomplexity of the filter arrangement so that each element of the filterconsistsfof an inductance and capacity in series with each other in theline, and of an inductance and a capacity in parallel to each other andin shunt to the line, a filter may be obtained which permits the passageof a band only as indicated in the curve of Fig. 3 Such a filter'hasbeen termed a band-pass filter and with suitable precautions the cut-0H"may be made: such as to give a selectivity far surpassing thatobtainable with tuned cir cuits. Fig. 3 indicates the structure for al'iandexclusion filter. In this case, the series elements comprise aninductance and capacity in parallel to each other, and the shuntelements comprise an inductance and capacity in series with each other.The characteristic then obtain-ed is as indicated by the curve of Fig.3, where frequency is again plotted as abscissae and attenuation asordinates.

In view of the large voltages of the transmitting-frequency present inthe circuit of the modulator 1 1, it is necessary to provide someprotection for the amplifier 24 and it is for this reason that we insertthe band p a band of width 25 andha-ving f, as. its

mid-value. If this f, is in the neighborhood of some tens of thousands,a filter can be constructed which is extremely selective, that is,

having extremely sharp cut-off points.

Obviously, the output from the filter 23 may pass directly to thedetector 26. However, as in the figure, we prefer to use an amplifierand an additional wave filter. The filter 23 in front of the amplifier24 serves the added function of protecting the amplifier from excessivepower which might possibly come in, of frequencies either above or belowthe band which is passed. At the same time, the band filter 25,following the amplifier, performs the function of suppressing anyadditional frequencies which may be developedin the circuit of the tube24 by virtue of the fact that it. is not a perfect amplifier. Whilet-helow-pass filter 27 is not absolutely essential in this circuit, wedesire to use it tosuppress such 'h1gh frequencies as will be generatedin thecircuit of the detector 26. If such oscillations were permitted topass, they would, in general, find sufficient unbalance in the coil 7 topermit the passage of such frequencies into the circuit 8 with thepossibility that the transmitting and receiving circuits will oscillateat some undesired frequency. With such a filter as 27, however, this isprevented'and the oscillations which do pass the filter will find a wellbalanced coil at 7, permitting the ready transmission of the signal 8 tothe remote end of the line.

It is evident that in case the message 8, is coming forward at the sametime as s, is received, modulated signals for s, will still betransmitted through the receiving circuit and be carried to the'speaker.It

will thus be evident to the speaker that the party at the other stationis endeavoring to speak. I

In Fig. 2 we have shown amodification of the circuit arrangement. Themessage 8, is, as before, impressed on a low-pass filter 9 and anamplifier 10. An oscillator 32, preferably of the thermionic tube type,is supplied to generate oscillations of the transmitting frequency f,,which ma in this case be taken as 525,000. A modu ating device 34 isassociated with the oscillator in any will be certain ones.

suitable manner, as, for example, inthe wellknown Heising shunt circuitwhich is shown in this figure. Theiamplified signal 8 from the amplifier10 is then impressed on'thc modulator 34 and as aresult thereexists inthe output circuit of the tube 32 modulated oscillations ofthefrequencyf, and 348,, whichmay then be radiated as before.

Incoming modulated messages on the car rier 7, are received on theantenna and picked up by acircuit36 tuned to the ca rrier f by whichthese oscillations may be impressed on the grid of the oscillator 32.There will then been modulation in the oscillator circuit of thelocallygenerated frequency f, and the received signal giving, amongother things, the intermediate frequencies f f "andf -f is Theseoscilations-of'intermediate frequency are transmited by means of thetransformer 37 to a receiving circuit which we have shown, as consistingof a low-pass filter 38, a band-pass filter 39, an amplifier 40, aband-pass filter 41,

a detector 42and alow-p'ass filter 43. These variouselements have thefollowing proper ties and. functions. The low-pass filter 38 has acut-off which may be in the neighbor hood of 30,000. The band-passfilter 39 has a mid-value of f f which in this case is 25,000, and thewidth of this filter is such as to just permit the passage of the band.f /"gis The filter 41 has the same properties as the filter 39. Thedetector42 may be of any suitable form, such as the thermionic type, anddetects the modulated intermediate frequency; down to the speech signal.The low-pass filter 43 transmits all frequencies below 2500 andsuppresses all others.

The low-pass filter'38 performs the same function as the band-exclusionfilter of Fig. 1. In general, it may be stated that filters which cutoff on one side only and which have fewer elements, as seen from Fig.3*, can be constructed to withstand higher volt ages and also withsharper cut-off characteristics. It is desirable, however, to followthis filter 38 with a band-pass filter to eliminate circuit of theoscillator 32, of which there The arrangement of two band-pass filterswith an amplifier between has the same desirable properties as indicatedin connection with Fig. 1.

Having now described our invention, what is claimed is:

1. In a duplex high frequency signaling system. a transmitting andreceiving circuit, a modulat-ing-demodulating circuit, associatedtherewithcomprising a vacuum tube, the plate circuit ofsaidtube'carrying the oscillations to be transmitted and the receivedoscillations and acting to clemodulate said received oscillations to anintermediate frequency, a band-excluding filter connected only.

with the said plate circuit to suppress the transmitting and receivingfrequencies and to pass the demodulated oscillations, a bandpassingfilter to pass the intermediate frequency, and a detector fordemodulating said passed intermediate frequency.

2. In a duplex radio telephone system, an antenna, a transmittingcircuit associated therewith, means for separating the transmitted andthe received signals comprising a filter adjustedto exclude thetransmitting and the received frequencies, and band filters in seriestherewith to pass an intermediate frequency produced by beating the re-I ceived frequencies with the transmitting carrier frequency.

3. In a duplex radio telephone system, a

transmitting and receiving antenna adaptedto transmit on one frequencyand receive on a different frequency, means for generating andimpressing modulated high frequency oscillations on said antenna, saidmeans com prising a vacuum tube circuit, said vacuum tube circuit havingmeans to receive and de- :modulate the incoming signals to an intera. Ina luplex high frequency telephone system, a transmitting and receivingcircuit,

a vacuum tube associated therewith for impressin'gon the circuit theoscillations to be transmitted and for demodulating the receivedoscillations, a receiving circuit associated with said tube, a filterfor suppressing the high voltage transmitting oscillat-1ons,a ndabandpass, filter for passingthe demodulated frequencies. I

5. In a duplex radio telephone systemadapted to transmit on onefrequency and re-v ceive on another frequency, a. transmitting andreceiving circuit, a Vacuumtube having a source of oscillationsassociated therewith for impressing on the circuit the oscillationsceived and transmitted frequencies and a band-pass filter for passlngthe intermediate frequency.

6. In a duplex radio telephone system adapted to transmit on, onefrequency and receive on a different frequency, an antenna,

a transmitting and receiving circuit, a vacuum tube having a sourceofoscillations associated therewith for impressing on the antenna theoscillations to be transmitted and for demodulating the receivedoscillations to an intermediate frequencyequal to the difference betweenthe transmitted and received frequencies, a receiving circuit associatedwith said. tube, a filter for suppressing the high voltage highfrequency oscillations, and a band-pass filter in series therewith forpassing the intermediate modulated frequency, and a detector fordemodulating said lastfrequency to the message frequency.

In testimony whereof, we have signed our names to this specificationthis14th day of August, 1922.

LLOYD ESPENSCI-IIED. DE LOSSK, MARTIN.

